Providing dimming capabilities to lighting applications saves energy and enhances ambiance. Thus, a substantial portion of the lighting infrastructure for both residential and commercial applications includes some form of dimming such as a phase-cut dimmer. Phase-cut dimmers function by isolating the light fixture from the AC mains during part of the AC cycle. If the AC main cycles at 60 Hz, the phase-cut dimmer would thus isolate the light fixture at a 120 Hz rate. The phase cutting is defined with regard to the zero crossing of the AC mains. If the phase cutting begins at the zero crossing, the resulting phase-cut dimmer is denoted as a leading-edge dimmer. If the phase cutting ends at the zero crossing, the resulting phase-cut dimmer is denoted as a trailing-edge dimmer.
An incandescent or fluorescent bulb may be directly powered by a phase-cut dimmer without any modification. But incandescent and fluorescent bulbs are being rapidly replaced by light emitting diode (LED)-based lighting solutions due to the improved efficiency, longer usable lifespan, and lack of toxic materials in LEDs. However, the replacement of incandescent (or fluorescent) bulbs by LEDs leads to some integration issues with regard to being powered through a phase-cut dimmer. Regardless of whether the phase-cut dimmer is a leading-edge or a trailing-edge dimmer, a conventional LED cannot typically be powered through a phase-cut dimmer without some adaptations. For example, an incandescent bulb filament cools relatively slowly and thus continues to output light during the periods in which the phase-cut dimmer isolates the incandescent bulb from the AC mains. In contrast, an LED fixture reacts very rapidly to the current isolation through the phase-cut dimmer. A conventional LED will thus be prone to flicker and other disconcerting issues if powered through a phase-cut dimmer without further adaptations. In addition, LED fixtures typically require a bleeder circuit to provide a sufficient latching current for a phase-cut dimmer such as a triac device.
As compared to the analog dimming applied through a phase-cut dimmer, LED fixtures are more readily dimmed through digital approaches in which the power-switch controller for the switching power converter for the LED applies the dimming. For example, the Zigbee alliance has promulgated a standard denoted as the Zigbee Light Link for wirelessly controlling the digital dimming of LED fixtures. But these digital approaches function as an alternative to phase-cut dimmers. The combination of conventional digital control of LED dimming and phase-cut dimming results in flicker, multi-firing, phase angle distortion, and other undesirable effects.
Accordingly, there is a need in the art for improved digital LED dimming techniques and systems that are compatible with the presence of a phase-cut dimmer.